1. Field of the Invention
The invention relates to the structure of elements used as parts of a drive power transfer belt wound on a pair of pulleys used in a belt-drive continuously variable transmission for vehicles and each having a variable-width groove.
2. Description of Related Art
A belt-drive continuously variable transmission is known which is provided on a power transfer path and which is constituted of a pair of pulleys each having a variable-width groove and a drive power transfer belt having a plurality of elements that are compressed at each pulley in a sandwiching manner and that are arranged in series on looped rings in the direction corresponding to the direction in which the thickness of each element is measured (will hereinafter be referred to as “element thickness direction” where necessary), the drive power transfer belt being wound on the pulleys so as to transfer power. According to such a drive power transfer belt, power is transferred between the pulleys via the drive power transfer belt with the elements pressed against each other in the element thickness direction. Therefore, if the opposed sides of the elements adjacent to each other are not sufficiently upright and parallel to each other when they are stacked on each other in the element thickness direction, an undesirable behavior or motion of each element may occur, resulting in noises and reduced efficiencies. Such a problem is described in Japanese Patent Application Publication No. 2003-247605. Further, Japanese Patent Application. Publication No. 2003-247605 describes elements for addressing the problem. Further, Japanese Patent Application Publication No. 2005-042803 includes some descriptions on the stress acting on the elements.
The elements described in Japanese Patent Application Publication No. 2003-247605 each have a recess formed at one side of the element and a projection projecting from the other side of the element, and the projection of each element engages with the recess of the element next to it. Further, the elements described in Japanese Patent Application Publication No. 2003-247605 each have a raised portion that is slightly raised from around the projection at the other side of the element. The ratio of the length over which the raised portion is raised from the other side of the element to the thickness of the element is 0.005 or lower. According to Japanese Patent Application Publication No. 2003-247605, the structure described above ensures that the opposed sides of the elements are kept upright sufficiently.
The projections of the respective elements described in Japanese Patent Application Publication No. 2003-247605 are generally columnar. Further, in belt-riding sections in which the drive power transfer belt wound on the pulleys are sandwiched by the respectively pulleys, the elements adjacent to each other incline relative to each other at a certain angle, as viewed in the axial direction of the pulleys, and for the purpose of allowing such relative inclinations of the elements adjacent to each other, the radial gap that is created between the projection and the recess when they are in engagement with each other is very large. For example, FIG. 13 schematically illustrates an example case where elements 510 are stacked on each other along the drive power transfer belt, as viewed from the radially outer side of the drive power transfer belt. In the example case illustrated in FIG. 13, if the diameter of each recess 512 is D1, the diameter of each projection 514 is D2, and the width of the radial gap between the projection 514 and the recess 512 is CLCD, the width of the radial gap CLCD is calculated as CLCD=D1−D2.
If the radial gap between the projection 514 and the recess 512 is very large, for example, the elements 510 stacked on each other can not be sufficiently inhibited from moving upward, downward, leftward, and rightward in directions parallel or substantially parallel to the opposed sides of the elements 510 in the belt-riding section at one of the pulleys and the belt-riding section at the other, and this results in undesirable behaviors and motions of the elements 510. Note that “belt-riding sections” represent sections in which the drive power transfer belt rides on the respective pulleys. The “undesirable behaviors and motions” of the elements 510 include, for example, an increase in the amount of pitching motion of each element 510 that pitches as viewed in the axial direction of the pulleys, and an increase in the amount of yawing motion of each element 510 that yaws as indicated by the arrows ARyw in FIG. 13, as viewed from the radially outer side or radially inner side of the drive power transfer belt.
When undesirable behaviors and/or motions of the elements 510 occur in either of the belt-riding sections, for example, the elements 510 slightly rub against each other at the position from which the drive power transfer belt leaves the pulley and at the position at which the drive power transfer belt comes to ride on the pulley, and this may cause an increase in the power loss (belt loss) that the drive power transfer belt causes during the power transfer through it. FIGS. 14A and 14B are views for illustrating, as one example of “undesirable behaviors and motions” of each element 510, how the amount of yawing motion of each element 510 increases in “belt-released sections” in which the drive power transfer belt are on neither of the two pulleys.
FIGS. 14A and 14B illustrate how the yaw angle of the element 510 described in Japanese Patent Application Publication No. 2003-247605, which is an example of related-art elements, changes over the entire circumference of a drive power transfer belt 516. With the drive power transfer belt 516 shown in FIGS. 14A and 14B, the drive power is transferred from a drive pulley (drive sheave pair) 520 to a driven pulley (driven sheave pair) 518 with the stacked elements 510 being pressed against each other. The chart of FIG. 14B shows that the more distant the value of the yaw angle is from the zero level, the larger the amount of yawing motion, regardless of which of the negative and positive sides the value of the yaw angle is in. The positions [1] to [4] at the horizontal axis of the chart in FIG. 14B correspond, respectively, to circumferential positions [1] to [4] at the drive power transfer belt 516 shown in FIG. 14A. More specifically, the circumferential position [1] is a driven pulley belt leaving position from which the drive power transfer belt 516 leaves the driven pulley 518 that is rotationally driven by the drive power transfer belt 516. The circumferential position [2] is a drive pulley belt entering position at which the drive power transfer belt 516 comes to ride on the drive pulley 520 that transfers drive power to the drive power transfer belt 516. The circumferential position [3] is a drive pulley belt leaving position from which the drive power transfer belt 516 leaves the drive pulley 520. The circumferential position [4] is a driven pulley belt entering position at which the drive power transfer belt 516 comes to ride on the driven pulley 518.
Referring to FIG. 14B, in the section from the circumferential position [3] to the circumferential position [4], the stacked elements 510 are pressed against each other for the drive power transfer. On the other hand, in the section from the circumferential position [1] to the circumferential position [2], the elements 510 are pressed against each other more lightly than they are in the section from the circumferential position (3) to the circumferential position (4), and therefore the amount of yawing motion of each element 510 is large in the section from the circumferential position [1] to the circumferential position [2]. In the section from the circumferential position [2] to the circumferential position [3], the yaw angle of each element 510 is made closer to the zero level by the drive pulley 520. That is, the positions of the respective elements 510 are straightened in the belt-riding section at the drive pulley 520. According to the related-art elements 510, for example, yawing motions of the elements 510 are not sufficiently restricted by the engagement between the projection 514 and the recess 512, and therefore each element 510 largely yaws at the drive pulley belt entering position (i.e., the circumferential position [2]), resulting possibly in an increase in the belt loss described above. Although the example case illustrated in FIGS. 14A and 14B is mainly for explanation on yawing of the elements 510, an increase in the amount of pitching motion of each element 510 may possibly cause an increase in the belt loss, etc.
It is to be noted that such problems have never been addressed in public.